Dynamic Enabling/disabling Of Form Elements?

I'm currently working on a form for my site and would like to do something
that I've seen many times on other sites. I would like to make a particular
form element enabled or disabled depending on whether another requisite form
radio button is checked. I tried this:

<input type="radio" name="yesorno" id="yes"
onClick="document.getElementById('metoo').disabled = 'false'">
<br>
<input type="checkbox" id="metoo" disabled="true">
<br>
<br>
<input type="radio" name="yesorno" id="no">

but it doesn't work--I can't figure out why.